Hundreds of passengers who were stranded on broken-down trains in tunnels under the Sydney CBD for more than an hour have had to walk through tunnels back to their nearest station.

Power supply issues hit Wynyard and Town Hall stations during peak hour on Tuesday, halting trains and leaving commuters crowding on platforms.

Commuters described "absolute chaos" as they descended into the underground stations just after 5.45pm to find them hot, crowded and packed with thousands of confused people.

One train lost power in the tunnels between Town Hall and Wynyard Stations, while another was stuck somewhere between Central and Town Hall.

Emma Veloska, from Hunters Hill, was one of the passengers on board the second train.

Ms Veloska told Fairfax Media it was "pitch black" and "boiling" in her carriage, as the power and air conditioning had been turned off. Many of the passengers were on their phones calling their relatives, while others were getting "fed up".

She said she had been waiting for more than an hour, with limited information from Sydney Trains on what would happen, until they were told just before 6.30pm via the train's intercom that they would have to walk to the platform at Town Hall station.

"They didn't tell us anything for ages, then they said that there was a power supply issue at Wynyard," she said. "I know we're going to have to get out of the train and walk through the tunnel the rest of the way."

As the evacuation began, she was ushered to the front of the train by Sydney Trains workers and escorted through the tunnels. When the passengers arrived at Town Hall station, Ms Veloska said they climbed through the door of a train that had stopped at the station and walked through that train until they emerged at the platform.

Paramedics and police, including officers from police rescue, were deployed at Wynyard Station in the early evening to intercept passengers expected to emerge from another stranded train.

Vision on social media showed passengers walking in a line in the darkness of a tunnel.

Train staff distributed water as the relieved passengers finally emerged onto the platform, some carrying small children, others sweaty and flustered.

A NSW Ambulance spokesman said no passengers were treated at the scene or taken to hospital.

Passengers described how the evacuation began at around 6.50pm, with babies and infants taken from the train first and passengers urged to remain calm as they made their way back to the station.

An announcement said there was a "physical problem with high voltage power lines" and the work to fix it would take "quite some time".

Julian Bowker, who was planning to catch the train from Wynyard to Umina Beach on the central coast, was at the station shortly after the power supply issues began. He said commuters were initially told there would be a delay, then just before 6pm they were told trains would remain at the station.

He described the station as being hot, humid and "packed", as there were no warnings on the surface of the power supply issue below, meaning people would take long escalators to below the surface then realise something was amiss.

Mr Bowker said he and many others left the station through the emergency exit rather than waiting for power to be restored.

Issues also beset Town Hall station, where there was crowding on platforms one and two. By the time the stranded passengers walked out into the station, one of the platforms was closed.

The Transport Management Centre said buses were replacing trains on the T1 North Shore & Northern Line from Wynyard to North Sydney as "urgent power supply repairs continue at Wynyard".

Passengers who needed to travel on the line were advised to allow "plenty of extra travel time" and listen to announcements at stations.

The power failure also had "flow-on delays" for the T1 Western and Blue Mountains line, Central Coast & Newcastle Line and both directions of the T2 Airport Inner West & South and T3 Bankstown lines.

All passengers were told to allow extra time for their journeys.

In a statement on Tuesday evening, Sydney Trains chief executive Howard Collins said he was sorry for the "major" delays.

"There have been extensive delays due to overhead wire damage between Town Hall and Wynyard stations," he said.

"This damage caused power issues, which has stopped all trains travelling away from the city between Central and North Sydney on the T1 North Shore and Northern lines since 5.30pm.

"Due to this incident, two trains were stopped between stations. Customers were safely evacuated from these trains and onto the platforms at Wynyard and Town Hall stations.

"We are working as hard as possible to fix the overhead damage and restore power to our services.